The thin film magnetic recording disk in a conventional hard disk drive assembly typically consists of a rigid substrate, an underlayer of chromium (Cr) or a Cr-alloy, a cobalt-based magnetic alloy deposited on the underlayer, and a protective overcoat over the magnetic layer. A variety of disk substrates such as NiP-coated Al--Mg, glass, glass ceramic, glassy carbon, etc., have been used. The microstructural parameters of the magnetic layer, i.e., crystallographic preferred orientation, grain size and magnetic exchange decoupling between the grains, play key roles in controlling the recording characteristics of the disk. The Cr underlayer is mainly used to control such microstructural parameters as orientation and grain size of the cobalt-based magnetic alloy. When the Cr underlayer is deposited at elevated temperature (&gt;150 C.) on a NiP-coated Al--Mg substrate a [200] preferred orientation (PO) is usually formed. This PO promotes the epitaxial growth of [1120] of the cobalt (Co) alloy, thereby improving the in-plane magnetic performance of the disk.
The use of glass substrates gives improved shock resistance and allows thinner substrates to be used. However, it is often observed that media fabricated on glass substrates have higher noise compared with those made on NiP-coated Al--Mg substrates under identical deposition conditions. The reason is that the nucleation and growth of Cr or Cr-alloy underlayers on glass and most non-metallic substrates differ significantly from those on NiP-coated Al--Mg substrate. It is for this reason that the use of an initial layer on the substrate called a seed layer has been proposed. The seed layer is formed between the alternate substrate and the underlayer in order to control nucleation and growth of the Cr underlayer and, therefore, the magnetic layers. Several materials have been proposed in the prior art as candidates for seed layers such as: Al, Cr, Ti, Ni.sub.3 P, MgO, Ta, C, W, Zr, AlN and NiAl on glass and non-metallic substrates. (See for example, Seed Layer induced (002) crystallographic texture in NiAl underlayers, Lee, et al., J. Appl. Phys. 79(8), April 1996, p.4902ff).
In order to control nucleation and growth of the Cr underlayer on glass (or alternate substrates), a variety of seed layers have been reported. H. Kataoka, et al., have reported that the deposition of a tantalum seed layer on glass substrates promotes the [200] orientation in the Cr underlayer which, in turn, promotes the [1120] orientation in the magnetic layer. (IEEE Trans. Mag. 31(6), Nov. 1995, p.2734ff). They compared Cr, Ta, W and Zr for use as seed layers using a fixed underlayer and magnetic layer. The magnetic alloy used in their study was a 27 nm thick ternary CoPtCr alloy. The underlayer was CrTi and was 100 nm thick. The purpose of adding Ti was to increase the lattice spacing for optimum matching with CoCrPt.
One method for improving the recording performance of a magnetic disk medium is the use of a CrTi underlayer, which was suggested by Michealsen, et al. in U.S. Pat. No. 4,245,008. Matsuda, et al., also reported that the addition of Ti to Cr increases the lattice parameters of the Cr to enhance the epitaxial growth of the magnetic layer. (J. Appl. Phys. 79, pp. 5351-53 (1996)). They have also reported that the grain size of CrTi underlayer decreases with increasing the Ti concentration. It should be noted that although sputtered Ti has usually a very small grain size, it is not suitable for use as an underlayer or a seed layer as it promotes the &lt;0001&gt; orientation in the magnetic layer, thereby making it unsuitable for longitudinal recording.
One quaternary alloy which has been proposed for use as a magnetic layer is CoPtCrB. A method of depositing the CoPtCrB alloy is specified in commonly assigned U.S. Pat. No. 5,523,173. The '173 patent describes sputter depositing a chromium or chromium alloy underlayer on a substrate in such a way to achieve a strong [200] crystallographic orientation of the underlayer to achieve a low noise, high coercivity medium. This orientation is achieved by depositing the underlayer on a negatively biased substrate under high temperature, low pressure conditions. The oriented underlayer prevents the subsequently deposited CoPtCrB alloy from orienting itself in its preferred, c-axis vertical orientation. The '173 patent specifies that the CoPtCrB alloy should comprise 4 to 12 at % platinum, 18 to 23 at % chromium and 2 to 10 at % boron. However, the optimum performance of the CoPtCrB on glass substrates is not easily obtained for two major reasons. Firstly, nucleation and growth of the Cr underlayer differ from those of the NiP coated AlMg substrates. In fact, Cr tends to grow with a [110] orientation on glass at T&lt;300 C. This is an undesirable orientation in that it can induce perpendicular anisotropy in the magnetic layer. Secondly, application of bias during deposition of Cr is problematic because glass and most alternative substrates are not conductive.